Behave WG T. Savolainen
Internet-Draft Nokia
Intended status: Standards Track J. Korhonen
Expires: June 21, 2012 Nokia Siemens Networks
December 19, 2011
Discovery of a Network-Specific NAT64 Prefix using a Well-Known Namedraft-ietf-behave-nat64-discovery-heuristic-04.txt
Abstract
This document describes a method for detecting presence of DNS64 and
for learning IPv6 prefix used for protocol translation on an access
network without explicit support from the access network. The method
depends on existence of a well-known IPv4-only domain name
"ipv4only.arpa". The information learned enables applications and
hosts to perform local IPv6 address synthesis and on dual-stack
accesses avoid traversal through NAT64.
Status of this Memo
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This Internet-Draft will expire on June 21, 2012.
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Internet-Draft NSP Discovery using WKN December 20111. Introduction
As part of the transition to IPv6 NAT64 [RFC6146] and DNS64 [RFC6147]
technologies will be utilized by some access networks to provide IPv4
connectivity for IPv6-only hosts. The DNS64 utilizes IPv6 address
synthesis to create local IPv6 presentations of peers having only
IPv4 addresses, hence allowing DNS-using IPv6-only hosts to
communicate with IPv4-only peers.
However, DNS64 cannot serve applications not using DNS, such as those
receiving IPv4 address literals as referrals. Such applications
could nevertheless be able to work through NAT64, provided they are
able to create locally valid IPv6 presentations of peers' IPv4
addresses.
Additionally, DNS64 is not able to do IPv6 address synthesis for
hosts running validating DNSSEC enabled resolvers, but instead the
synthesis must be done by the hosts themselves. In order to perform
IPv6 synthesis hosts have to learn the IPv6 prefix(es) used on the
access network for protocol translation.
This document describes a best effort method for applications and
hosts to learn the information required to perform local IPv6 address
synthesis. An example application is a browser encountering IPv4
address literals in an IPv6-only access network. Another example is
a host running validating security aware DNS resolver in an IPv6-only
access network.
The knowledge of IPv6 address synthesis taking place may also be
useful if DNS64 and NAT64 are present in dual-stack enabled access
networks. In such cases hosts may choose to prefer IPv4 in order to
avoid traversal through protocol translators.
It is important to notice that use of this approach will not result
in as robust and good behaving system as an all-IPv6 system would be.
Hence it is highly RECOMMENDED to upgrade to IPv6 and utilize the
described method only as a short-term solution.
2. Requirements and Terminology2.1. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Internet-Draft NSP Discovery using WKN December 20112.2. Terminology
Well-Known IPv4-only Name (WKN): a fully qualified domain name,
"ipv4only.arpa", well-known to have only A record.
Well-Known IPv4 Address: an IPv4 address that is well-known and
mapped to the well-known name.
3. Host behavior
A host requiring information about presence of NAT64 and the IPv6
prefix used for protocol translation SHALL send a DNS query for AAAA
records of a well-known IPv4-only fully qualified domain name:
"ipv4only.arpa". The host MAY also need to perform DNS query for the
A record of the well-known name in order to learn what is the IPv4
address of the well-known name. The host may perform this check in
both IPv6-only and dual-stack access networks.
When sending AAAA query for the well-known name a host MUST set
"Checking Disabled (CD)" bit to zero, as otherwise the DNS64 will not
perform IPv6 address synthesis hence does not reveal the IPv6
prefix(es) used for protocol translation.
A DNS reply with one or more non-empty AAAA records indicates that
the access network is utilizing IPv6 address synthesis. A host MUST
look through all of the received AAAA records to collect all
available prefixes. The prefixes may include Well-Known Prefix 64:
ff9b::/96 [RFC6052] or one or more Network-Specific Prefixes. In the
case of NSPs the host SHALL search for the IPv4 address of the well-
known name inside of the received IPv6 addresses to determine the
used address format.
An IPv4 address of the well-known name should be found inside
synthetic IPv6 address at some of the locations described in
[RFC6052]. If the searched IPv4 address is not found on any of the
standard locations the network must be using different formatting.
Developers may over time learn on IPv6 translated address formats
that are extensions or alternatives to the standard formats.
Developers MAY at that point add additional steps to the described
discovery procedures. The additional steps are outside the scope of
the present document.
The host should ensure a 32-bit IPv4 address value is present only
once in an IPv6 address. In case another instance of the value is
found inside the IPv6, the host shall repeat the search with another
IPv4 address, if possible.
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Internet-Draft NSP Discovery using WKN December 2011
In the case only one IPv6 prefix was present in the DNS response: a
host shall use that IPv6 prefix for both local synthesis and for
detecting synthesis done by the DNS64 entity on the network.
In the case multiple IPv6 prefixes were present in the DNS response:
a host SHOULD use all received prefixes when determining whether
other received IPv6 addresses are synthetic. However, for selecting
prefix for the local IPv6 address synthesis host MUST use the
following prioritization order, of which purpose is to avoid use of
prefixes containing suffixes reserved for the future [RFC6052]:
1. Use NSP having /96 prefix
2. Use WKP prefix
3. Use longest available NSP prefix
In the case of NXDOMAIN response or an empty AAAA reply: the DNS64 is
not available on the access network, network filtered the well-known
query on purpose, or something went wrong in the DNS resolution. All
unsuccessful cases result in unavailability of a host to perform
local IPv6 address synthesis. The host MAY periodically resend AAAA
query to check if DNS64 has become available or possibly temporary
problem cleared. The host MAY perform A query for the well-known
name to learn whether the NAT64 prefix discovery framework is
available at all (see section 6 about Exit Strategy). The host MAY
also continue monitoring DNS replies with IPv6 addresses constructed
from WKP, in which case the host MAY use the WKP as if it were
learned during the query for well-known name.
To save Internet's resources, if possible, a host should perform
NAT64 discovery only when needed (e.g. when local synthesis is
required, cached reply timeouts, new network interface is started,
and so forth. Furthermore, the host SHOULD cache the replies it
receives and honor TTLs.
3.1. Learning NAT64 prefix securely by using DNSSEC
If a node is using untrusted channel between itself and DNS64, or
DNS64 entity itself is untrusted, it is possible for an attacker to
influence node's NAT64 prefix detection procedures. This may result
in denial-of-service, redirection, man-in-the-middle, or other
attacks. To protect against these attacks the node may use DNSSEC,
or communicate with trusted DNS64 over trusted channel.
Significantly, explicit access network support is needed for the
DNSSEC approach to work.
To support DNSSEC capable nodes to perform NAT64 prefix learning
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Internet-Draft NSP Discovery using WKN December 2011
securely, NAT64 using access network MUST do the following. In the
case of multiple IPv6 prefixes being used in a network, e.g. for
load-balancing purposes, it is for network administrators to decide
whether a single NAT64's fully qualified domain name maps to multiple
prefixes, or whether there will be dedicated FQDN per IPv6 prefix.
1. Have one or more fully qualified domain names for the NAT64
translators (NAT64 FQDN).
2. Have each NAT64 FQDN to have one or more DNS AAAA records with
each IPv6 address consisting of NAT64 prefix and 0's for the
elements after the actual NAT64 prefix.
3. Sign the NAT64 FQDNs' AAAA records with DNSSEC.
4. Have access network's authoritative nameservers to respond to DNS
queries for the NAT64 FQDNs only when the queries have been
originated from the network domain the NAT64 is serving. If the
NAT64's AAAA records are made resolvable throughout the Internet,
a possible misuse vector of the NAT64 prefixes and NAT64 FQDNs in
other networks is enabled: an attacker in other access network
may lure a host on that network to think it is configuring NAT64
prefix in secure manner, while in reality it is not as the node
would be configuring NAT64 prefix in a network where the NAT64
prefix should not be used.
A DNSSEC capable node MUST use the following procedure to confirm the
learned NAT64 prefix is legitimate:
1. Node MUST heuristically find out a NAT64 prefix candidate by
making AAAA query for the "ipv4only.arpa".
2. Node MUST send DNS PTR query for the IPv6 address of the
translator (for "ipv6.arpa"), using the prefix from the step 1
and 0 for the elements after the prefix length. This will return
the NAT64 FQDN.
3. Node MUST send DNS AAAA query for the NAT64 FQDN.
4. Node MUST verify the DNS AAAA response matches the address
obtained in step 1. It is possible that the NAT64 FQDN maps to
multiple AAAA records, in which case the node has to check if any
of the responses matches to the address obtained in step 1. The
node MUST ignore other responses and not to use those for local
IPv6 address synthesis.
5. Node MUST perform DNSSEC validation of the DNS AAAA response.
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After the node has used DNSSEC to validate the IPv6 address received
as response to the NAT64 FQDN query the node can consider NAT64
prefix securely learned.
3.2. Connectivity test
After a host has obtained a candidate prefix and format for the IPv6
address synthesis, the host may locally synthesize an IPv6 address,
by using a publicly routable IPv4 address, and test connectivity with
the resulting IPv6 address. The connectivity test may be conducted
e.g. with ICMPv6 or with a transport layer protocol.
This connectivity test ensures local address synthesis results in
functional and protocol translatable IPv6 addresses.
The host MUST NOT perform connectivity test against the IPv4 address
of the well-known name, but instead use some other destination such
as host vendor's servers.
In many scenarios separate connectivity test is not really required
as an application may just try to connect to the IPv4-only
destination with synthetic IPv6 address and see if a connection is
successfully established or not.
4. Operational considerations for hosting the IPv4-only well-known name
The authoritative name server for the well-known name shall have DNS
record TTL set to a long value in order to improve effectiveness of
DNS caching and robustness of the discovery procedure in general.
The exact value depends on availability time for the used public IPv4
address, but should not be longer than one year.
The domain serving the well-known name must be signed with DNSSEC.
See also Security Considerations section.
It is expected that volumes for well-known name related queries are
roughly SOMETHING, TBD. The infrastructure required to serve well-
known name is SOMETHING, TBD.
5. DNS(64) entity considerations
DNS(64) servers MUST NOT interfere or perform special procedures for
the queries related to the well-known name until the time has arrived
for the exit strategy to be deployed.
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Internet-Draft NSP Discovery using WKN December 20116. Exit strategy
A day will come when this tool is no longer needed. At that point
best suited techniques for implementing exit strategy will be
documented. In the global scope the exit strategy may include
sending NXDOMAIN replies by the authoritative name server of the
well-known name with a very long TTL.
A client implementation receiving NXDOMAIN response for the A query
of the well-known name means SHOULD consider this tool as temporarily
disabled.
7. Security Considerations
The security considerations follow closely those of RFC6147
[RFC6147]. If an attacker manages to change the NAT64 prefix host
discovers, the traffic generated by the host will be delivered to
altered destination. This can result in either a denial-of-service
(DoS) attack (if the resulting IPv6 addresses are not assigned to any
device), a flooding attack (if the resulting IPv6 addresses are
assigned to devices that do not wish to receive the traffic), or an
eavesdropping attack (in case the altered NSP is routed through the
attacker).
The zone serving the well-known name has to be protected with DNSSEC,
as otherwise it will be too attractive target for attackers who wish
to alter hosts' NSP prefix discovery procedures.
A host SHOULD implement validating DNSSEC resolver for validating the
A response of the well-known name query. A host without validating
DNSSEC resolver SHOULD request validation to be performed by the used
recursive DNS server.
For the secure NAT64 prefix discovery the access network SHOULD sign
the NAT64 translator's fully qualified domain name, and make that DNS
resolvable only from the network domain NAT64 is serving. Otherwise
the NAT64 prefix may be used for attacks in other access networks. A
host SHOULD use the algorithm described in Section 3.1 in order to
securely learn the NAT64 prefix.
8. IANA Considerations
According to procedures described in RFC3172 this document requests
IANA and IAB to reserve a second level domain from the .ARPA zone for
the well-known domain name. The well-known domain name could be, for
example, "ipv4only.arpa".
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Internet-Draft NSP Discovery using WKN December 2011
The well-known name also needs to map to one but preferably to two
different public IPv4 addresses.
8.1. About the IPv4 address for the well-known name
The IPv4 address for the well-known name, if possible, should be
chosen so that it is unlikely to appear more than once within an IPv6
address and also as easy as possible to find from within the
synthetic IPv6 address. An address not listed in the Section 3 of
[RFC5735] is required as otherwise DNS64 entity may not perform AAAA
record synthesis. The address does not have to be routable or
allocated to any node, as no communications are initiated to the IPv4
address.
Allocating two IPv4 addresses would improve the heuristics in cases
where the primary IPv4 address' bit pattern appears more than once in
the synthetic IPv6 address (NSP prefix contains the same bit pattern
as the IPv4 address).
If no well-known IPv4 address is statically allocated for this
method, the heuristic requires sending additional A query to learn
the IPv4 address that is sought inside the received IPv6 address.
Without knowing IPv4 address it is impossible to determine address
format used by DNS64.
9. Acknowledgements
Authors would like to thank Andrew Sullivan, Washam Fan, Cameron
Byrne, Zhenqiang Li, Dave Thaler, Peter Koch, and Christian Huitema
for significant improvement ideas and comments.
Special acknowledgements go to Dan Wing for the NAT64 prefix secure
discovery algorithm.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
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